Radiation dosimeters which use insulated gate field effect transistors are well known. Some such dosimeters comprise a "floating gate" interposed between a control gate and the channel between the source and the drain. When the dosimeter is in use, the ionizing radiation changes the charge on the floating gate which, in turn, changes the threshold voltage of the transistor. The change in the threshold voltage is a measure of the amount of ionizing radiation to which the dosimeter has been exposed.
Some floating gate dosimeters are "active", in that they require biasing in order for the floating gate charge to be changed by ionizing radiation. Such a floating gate dosimeter has been disclosed by Knoll et al in U.S. Pat. No. 4,788,581 issued November 1988. The floating gate is provided between the usual control gate and the channel between the source and the drain. In the method of operation disclosed by Knoll et al, the floating gate is initially uncharged. To make the dosimeter sensitive to radiation, a bias voltage must be applied to the control gate. Charge generated by radiation in the insulating layers of the floating gate structure moves in response to this control gate bias, and part of the charge is collected on the floating gate. The presence of charge on the floating gate alters the MOSFET electrical characteristics, which allows the absorbed radiation dose to be determined. An experimental demonstration of this method of operation of a floating gate MOSFET dosimeter has been reported by Peters et al in an article entitled "A floating gate MOSFET gamma dosimeter", Can. J. Phys., 74, S685 (1996).
The need for a power supply and biasing limits the application of "active" devices. Where the detector must be attached to or inserted into a patient being treated with radiation; used in space craft where power consumption must be minimized; or attached to a space suit worn by an astronaut during extra-vehicular activity; or attached to the gloved hands of persons handling radioactive materials, for example, it is preferable to use a "passive" dosimeter, which has its floating gate charged before exposure to the radiation. Irradiation causes the charge on the floating gate to change, causing the threshold level of the device to change also. Following irradiation, the "passive" device is connected to a circuit which applies bias and measures the electrical characteristics to determine the change in charge level and hence the amount of radiation to which the device was exposed.
Examples of "passive" dosimeters are disclosed in U.S. Pat. No. 5,596,199 issued January 1997 naming McNulty et al as inventors, in international patent application number WO 95/12134 published May 1995 naming J. Kahilainen as inventor, and in an article entitled "Radiation Dosimeter Based on Floating Gate MOS Transistor" by Kassabov et al in Radiation Effects and Defects in Solids, 1991, Vol. 116, pp. 155-158. Insulated gate field effect transistors with floating gates are also used in EEPROMs, as disclosed at the Canadian Conference on Very Large Scale Integration, Banff, Nov. 14-16, 1993 by G. C. McGonigal and H. C. Card in a disclosure entitled "Analog EEPROMs with Low Programming Voltage for Adaptive Circuitry in Northern Telecom CMOS4S 1.2 .mu.m Technology". Although Messrs. McGonigal and Card were concerned with providing VLSI designers with a variety of adaptive, non-volatile, analog and digital functions, such as neural network synaptic weights and high-precision circuit trimming, and did not suggest using the device for radiation measurement, they described charging of the floating gate of an insulated gate field effect transistor having a floating gate, a control gate and an injector gate.
Kahilainen (WO 95/12134) describes a floating gate dosimeter without a control gate in addition to the floating gate. The floating gate is charged by applying a sufficiently high voltage between the source and drain to cause tunnelling to occur through the oxide layer of the gate insulator. The other devices each have a floating gate interposed between a control gate and the channel. Thus, Kassabov et al charge the floating gate by applying voltage impulses to the control gate. Likewise, McGonigal and Card charge the floating gate by applying "programming pulses" to the control gate. McNulty et al charge the floating gate by applying a negative voltage between the source and the drain such that electrons from the drain are "swept up" to the floating gate by the more positively charged control gate, which is held at a constant voltage.
A disadvantage of these known methods of charging the floating gate is that they may result in interface states which can give noise and long-term stability problems, particularly in view of the relatively high sensitivity involved in most radiation measurements.
A further disadvantage of known passive dosimeters is their susceptibility to temperature variations. The threshold voltage V.sub.T of a MOSFET varies in dependence upon temperature, which is of concern for a MOSFET dosimeter, since a change in V.sub.T in response to temperature variation could be falsely interpreted as an indication of exposure to radiation. It is known to operate a pair of active MOSFET dosimeters differentially to compensate for temperature variations. Thus, U.S. Pat. No. 4,678,916 issued July 1987, naming I. Thomson as inventor, discloses a dosimeter comprising a pair of insulated gate field effect transistors integrated into the same substrate but biased to different levels during radiation measurement. The output of the dosimeter is the difference between the threshold voltages of the two MOSFETs. Both threshold voltages will be affected to substantially the same degree by temperature variations, but the difference will be substantially unaffected. Although this approach has been used with active dosimeters, the methods of charging the floating gates of the above-mentioned known passive dosimeters are too imprecise for satisfactory differential operation.
An object of the present invention is to eliminate or at least ameliorate the disadvantages of the known method of monitoring radiation using passive floating gate dosimeters and to provide an improved method of monitoring radiation and a dosimeter for use therein.